Coaxial to elliptical waveguide coupling



Dec. 20, 1966 KRANK ETAL 3,293,573

COAXIAL TO ELLIPTICAL WAVEGUIDE COUPLING Filed March 25. 1965 v 2 Sheets-Sheet 1 Fig. 1

IN VENTORS Wolfgang Monk 8 Dieter Rumonqt BYQ%mcW 5 W ATTORNEYS Dec. 20, 1966 Filed March W. KRANK ET AL COAXIAL T0 ELLIPTICAL WAVEGUIDE COUPLING 2 Sheets-Sheet 2 BANDWITH WHERE r=0.02

IO- l I l l vswR j INVENTORS Wolfgang Krcmk 8 v Dieter Rumoncn FIG.4.

ATTORNEYS United States Patent 3,293,573 COAXIAL T0 ELLIPTICAL WAVEGUIDE COUPLING Wolfgang Krank and Dieter Ramonat, both of Backnang,

Germany, assignors to Telefunken Patentverwertungsgesellschaft m.b.H., Ulm (Danube), Germany Filed Mar. 25, 1965, Ser. No. 442,649

Claims priority, application Germany, Mar. 25, 1964,

T 25,900 16 Claims. (Cl. 33321) The present invention relates generally to the microwave art, and, more particularly, to an arrangement for the excitation of a wave in a waveguide which is of generally elliptical cross section and has a short-circuit wall at the end thereof which serves for the central couplingin of a coaxial line.

Up to now, connections in which the coupling-in of energy is provided in the direction of the major axis of the waveguide have been made as inductive coupling loops. In an article by G. J. Wheeler, IRE Convention Record, Part 1, 1957, difierent types of couplings are described which are constructed according to this principle. Such couplings have the characteristic that the reflection factor almost suddenly reaches very high values at the band limits, which results in this type of arrangement being usable for only relatively small bandwidths.

With this prior art in mind it is a main object of the present invention to provide a coupling arrangement which does not have the above-mentioned disadvantages.

Another object of the invention is to provide a device of the character described which can be used for larger bandwidths than previously.

Another object of the invention is to construct such a device which is primarily capacitive.

These objects and others ancillary thereto are constructed in accordance with preferred embodiments of the invention wherein a waveguide having a short-circuited wall is provided with a cross bar at a predetermined distance from the short-circuited wall. This cross bar is provided with thickened portions and is conductively connected with the inner conductor of a coaxial line. The waveguide is generally elliptical and the cross bar is parallel to the major axis thereof while the ends of the cross bar are connected to the waveguide wall. A further metal cross piece is mounted centrally and at a right angle to the inner conductor of the coaxial line and to the cross bar. One end of this metal cross piece is connected with the wall of the waveguide and the other end protrudes slightly past the cross bar.

Additional objects and advantages of the present invention will become apparent upon consideration of the fol- I lowing description when taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a sectional view taken through a waveguide in which the coupling of the present invention is provided.

FIGURE 2 is a longitudinal sectional view of the device, illustrated in FIGURE 1.

FIGURE 3 shows the characteristical curve for the dependence of the relative bandwidth raj/13.100 (in percent) from the relationship d/ b. d means the enlarged portion and b the minor axis of the elliptical cross section of the wave guide.

FIGURE 4 shows the curve of VSWR in dependence of c/ b. c is the normal diameter of the metallic cross bar.

With more particular reference to the drawings, a wave guide 2 is provided which is of elliptical or elliptical-like cross section. The end of the waveguide has a shortcircuit wall which serves for the coupling-in of the coaxial line. Furthermore, this short-circuit wall is provided with a central bore so that the inner conductor of the coaxial line can pass into the waveguide. The view of FIGURE 1,

3,293,573 Patented Dec. 20, 1966 which is a cross section, is taken in the coupling zone of the waveguide and it can be seen from FIGURE 2, for example, that the coupling element is disposed at a distance l from the short-circuit wall. The distance 1 is between AH/ 8 and AH/ 16 and preferably amounts to AH/ 16, where )H designates the wave length of the waveguide.

The inner conductor 1 of the coaxial line is introduced centrally into the waveguide 2 and is conductively or galvanically connected to the coupling arrangement. The coupling arrangement includes a cross bar 4 which is provided with thickened portions 8, 13 and 14. The cross bar is parallel to the major axis of the elliptical Wave guide 2 and at its ends is conductively connected to the waveguide wall 16. At right angles to the inner conductor 1 and at a right angle to the metallic cross bar 4 is a further metal cross piece 5 which is connected galvanically to the waveguide at one end thereof and at the other end thereof protrudes slightly past the cross bar 4 as indicated at B.

The cross connection represented by the cross bar 4 has the same effect in the coupling plane as if the waveguide cross section were loaded by a capacitive slot. Therefore, the mechanical length between the cross connection and the short-circuit wall is smaller than AH/ 4 and preferably amounts to )\H/l6. Within the coupling zone, the coaxial line can be considered as a special heterogeneous coaxial line which transfers the electro-magnetic field into the waveguide whereby the characteristic impedance of the coaxial cable is transformed into the characteristic impedance of the waveguide.

The projection 6 of the metal cross piece 5 which is parallel to the minor waveguide axis provides for the excitation of the desired waveguide wave because the lines of force of the type of wave which is to be excited in the waveguide takes its course in the direction of the exciting element. The combination of the cross bar 4 with the metal cross piece 5 provides a coupling arrangement which, on the whole, has a prevailing capacitive component.

In order to be able to obtain as large a bandwidth as possible with this arrangement, enlarged or thickened portions are provided on the cross bar 4. Such an enlarged portion 14 is located at the central connection or meeting point of the cross bar 4 and the metal cross piece 5. Two further thickened portions 3 are located symmetrically to this and the distance 1 between them is about )\H/ 4.

It has proven to be advantageous to produce the ends of the cross bar 4 which are connected conductively with the waveguide wall, to be thicker than the normal diameter c of the cross bar 4. A further improvement of the electrical characteristics of the arrangement can be obtained by providing that the length of the smaller cross section portions 10 located between the center enlargement 14 and the two enlargements 8, positioned symmetrically to the center, is about xH/ZO each. The diameter of the enlargements 8, 13 and 14 have been chosen in the illustrated embodiment in such a manner that the following relationship was provided:

The term b designatesthe small diameter of the waveguide 2. The best relationship of d :b was 0.25. Experiments which have been conducted have shown that optimum ratios regarding the bandwidth are obtained after the ratio of the smallest diameter 0 of the cross bar and the smallest diameter b of the waveguide amounted to 0.15.

The inner conductor 1 of the coaxial line which is connected with the cross bar 4 can also be suitably provided with an enlargement 9, the size of which is between that of the central enlargement 14 and the normal diameter of the inner conductor 1, and this enlargement 9 is provided over the length thereof which projects into the waveguide 2. This enlargement operates together with the short-circuit wall 7 as a transforming element for the input resistance. The length 1 of the metal cross piece 5 including its end 6 which projects beyond the horizontal plane passing through the major axis amounts to )\H/ 4.

As can be seen in FIGURE 2, the distance between the connection point of the cross bar and the metal cross piece 5 and the short-circuit wall 7 is designated and preferably amounts to )\H/16. Generally the inner conductor 1 does not protrude past the metal cross piece 5. However, for special applications it can be of advantage to have the end 3 of the inner conductor 1 protrude slightly past the metal cross piece 5 and project into the waveguide 2.

In order to adjust electrically for slight differences caused by production tolerances, and in order to provide a simple manner for the electrical adjustment in general, two adjusting screws 11 and 12 are provided in the wall of the waveguide before the metal cross piece 5, and the distance between these adjusting screws amounts to AH/ 8. The distance of the screw 12 from the coupling plane preferably would be about )rH/ 8. The outer conductor 15 of the coaxial line does not enter into the waveguide but ends at the short-circuit wall 5 with which it is conductively connected.

With an arrangement constructed in accordance with the present invention a frequency bandwidth of 1:115 can be obtained based on a reflection factor of r: 0.045. With a relative reflection factor of r=0.02 a relative frequency bandwidth of i8% can be obtained, whereas the conventional coupling device only provided a bandwidth of :L-2%.

FIGURE 3 show-s that the optimum value for d/ b is 0.25 and referred to the limit of the relative bandwidth Af/f of 8% the characteristic reaches the limiting values at 0.15 and 0.3 for d/b. The curve is unsymmetric in view of the optimum value d/b=0.25.

The curve of FIGURE 4 has an absolute minimum at c/b=0.15 and shows a symmetrically characteristic. Naturally for another value than c/b=0.15 not only the VSWR increases but also the relative bandwidth Af/ referred to the limit of VSWR=-1.04 becomes lower.

It will be understood that the above description of the present invention is susceptible to various modifications, changes, and adaptations, and the same are intended to be comprehended within the meaning .and range of equivalents of the appended claims.

What is claimed is:

1. In an arrangement for the excitation of a wave in a generally elliptical waveguide having a short circuit end wall for centrally coupling a coaxial line thereto, the improvement comprising: a cross bar connected at its ends to the wall of the waveguide and having enlarged portions and conductively connected with the inner conductor of such coaxial line, said cross bar being disposed at a predetermined distance from the short circuit wall and parallel to the major cross sectional axis of the waveguide; and a centrally disposed metal cross piece positioned at a right angle to the inner conductor and tothe cross bar, one end thereof being connected to the wall of the waveguide and the other end protruding slightly beyond said cross bar.

2. An arrangement for the excitation of a wave in a waveguide, comprising, in combination:

(a) a generally elliptical waveguide defined by a first wall and short cirouited at one end by a second wall;

(b) a coaxial line having i311 inner and an outer conductor centrally coupled to said waveguide through said second wall;

() a cross bar connected at its ends to the first waveguide wall at a distance from said second wall and having enlarged portions, said cross bar being conductively connected with the inner conductor of said coaxial line and disposed parallel to the major cross sectional axis of the waveguide; and

(d) a centrally disposed cross piece positioned at a right angle to the inner conductor .and to the cross bar, to which it is connected, one end of said cross bar being connected to the first wall and the other end thereof protruding slightly beyond said cross bar.

3. An arrangement as defined in claim 2 wherein the distance between the zone where coupling takes place and the short circuit wall is between 7\H/8 and XH/ 16 in length, where AH designates the waveguide wavelength.

4. An arrangement as defined in claim 3 wherein said cross bar has a center enlarged portion, and two further enlarged portions arranged symmetrically to the center enlarged portion and spaced )rH/ 4 apart.

5. An arrangement as defined in claim 4 wherein the length of the cross piece is about kH/4.

6. An arrangement as defined in claim 5 wherein the reduced portions of the cross bar between the center enlarged portion and the further enlarged portions have a length of about xH/ZO.

7. An arrangement as defined in claim 6 wherein the further enlarged portions have a length of about kH/ZO.

8. An arrangement as defined in claim 4 wherein the ends of the cross bar are also provided with enlarged port-ions.

9. An arrangement as defined in claim 4 wherein the diameter of the enlarged portions d and the minor axis 12 of the elliptical cross section of the waveguide have the following relationship:

10. An arrangement as defined in claim 3 wherein the smallest diameter c of the cross bar and the minor axis b of the elliptical waveguide cross section have about the following relationship:

11. An arrangement as defined in claim 2 wherein the distance between the zone where coupling takes place and the short circuit wall is AH/ 16, where AH designates the waveguide wavelength.

12. An arrangement as defined in claim 2 wherein the ends of the cross bar are provided with enlarged portions.

13. An arrangement as defined in claim 2 wherein the diameter of the enlarged portions d and the minor axis b of the elliptical cross section of the waveguide have the following relationship: t

14. An arrangement as defined in claim 13 wherein d/b is equal to 0.25.

15. An arrangement as defined in claim 13 wherein the smallest diameter 0 of the cross bar and the minor axis b of the elliptical waveguide cross section have about the following relationship:

16. An arrangement as defined in claim 2 wherein two adjustment screws are provided in the first wall opposite the protruding portion of the cross piece, said screws being spaced XH/ 8 apart, and the distance between the cross bar and the nearest screw being kH/8, where AH is the waveguide wavelength.

No references cited.

HERMAN KARL SAALBACH, Primary Examiner.

L. ALLAHUT, Assistant Examiner. 

1. IN AN ARRANGEMENT FOR THE EXCITATION OF A WAVE IN A GENERALLY ELLIPTICAL WAVEGUIDE HAVING A SHORT CIRCUIT END WALL FOR CENTRALLY COUPLING A COAXIAL LINE THERETO, THE IMPROVEMENT COMPRISING: A CROSS BAR CONNECTED AT ITS ENDS TO THE WALL OF THE WAVEGUIDE AND HAVING ENLARGED PORTIONS AND CONDUCTIVELY CONNECTED WITH THE INNER CONDUCTOR OF SUCH COAXIAL LINE, SAID CROSS BAR BEING DISPOSED AT A PREDETERMINED DISTANCE FROM THE SHORT CIRCUIT WALL AND PARALLEL TO THE MAJOR CROSS SECTIONAL AXIS OF THE WAVEGUIDE; AND A CENTRALLY DISPOSED METAL CROSS PIECE POSITIONED AT A RIGHT ANGLE TO THE INNER CONDUCTOR AND TO THE CROSS BAR, ONE END THEREOF BEING CONNECTED TO THE WALL OF THE WAVEGUIDE AND THE OTHER END PROTRUDING SLIGHTLY BEYOND SAID CROSS BAR. 